We propose continuation of the development and exploratory application of a new biochemical assay method which combines the high sensitivity and speed of mass spectrometry with the specificity of enzymes. By localization of an enzyme catalyzed reaction near or upstream from a semi-permeable membrane, which allows transfer of volatile molecules into the mass spectrometer, any reactant in an enzyme catalyzed reaction can be assayed if at least one of the other reactants is reasonably volatile. Thus, a large number of substrates, cofactors, effectors such as inhibitors, and also enzymes themselves, can be assayed. Although current work necessarily focuses on apparatus modification and improvement, the following enzymes have been successfully tried to date: 1) Urease, 2) alcoholdehydrogenase, 3) catalase, 4) DOPA decarboxylase and 5) acetylcholinesterase, and a great many more appear suitable. BIBLIOGRAPHIC REFERENCE: "Possible Biomedical Applications of the Volatile Enzyme Product Method", James C. Weaver, in Biomedical Applications of Immobilized Enzymes and Proteins, T.M.S. Chang, Ed., Plenum Press, pp. 207-225 (1977).